RtHW Stat Blocks in CoS.

With the release of the new Ravenloft book, there are new stat blocks for Ez D'Avenir and Rudolph Van Richten. Both of these characters are in the CoS, and can be destined allies to help you fight Strahd. Although, both of these (especially Ez) have very powerful actions and attacks. I'm worried that if I use these in the CoS, they could potentially carry some encounters by themselves. Any advice? 

Ally doesn't mean that they actually fight, just that they provide some sort of assist. They're not even the most powerful possible allies (that would be the mad wizard), though they are potentially available much earlier.

I recently took a party of 5 through CoS. They had a bit of the completionist bent, and managed to collect Van Richten, Ezmerelda, and Kasimir to boot. By the time they were level 8, they might have wiped the boss fight easily, but thankfully they didn't know that... I just had the allies not take leadership during events. There were plenty of ways the PCs might have planned to have the allies use their powers, but they were more focused on their own exploits, so it all worked out in the end. I recommend a similar arrangement. The allies use powers and abilities when requested to do so by a player with agency... It doesn't have to be a strict rule... What was nice was the allies gave me permission to be as evil as possible with Strahd. If the fight had gone against them, there was a way to save the day...

Npc's tag along with the party in most campaigns.

They generally act as

backup characters (if pc dies, player can run an npc until original charavter is conscious again)

Support characters (if pc goes to 0 hp and making death saves, npc can cast healing word on them)

And sometimes fire support character (if characters can get monster below 100hp, the wizard npc will cast finger of death, or whatever, to finish them off. If the party walks into a room with 100 goblins, npc wizard will cast fireball, etc)

Sometimes the dm sets up an explicit game mechanic for an encounter. This npc has 3 points that can be spent on faerie fire or something else or another thing. And then each player manages one of the npcs.